There may not be life on the surface of Mars at the present, this does not exclude the possibility that life may have thrived earlier on the Red Planet. The early history of Mars seems to have been very similar to that of Earth, especially with respect to the ancient hydrosphere. The sedimentary structures in the Gillespie Lake Member, Mars, constitute a promising set of potential biosignatures that compel further analyses by Mars rovers, including future sample return missions from Mars. The sandstone structures on Mars look like structures created by microbes on earth.
Sandstone beds of the less than 3.7 billion years Gillespie Lake Member on Mars have been interpreted as evidence of an ancient playa lake environment. On Earth, such environments have been sites of colonization by microbial mats from the early Archean to the present time. Terrestrial microbial mats in playa lake environments form microbialites known as microbially induced sedimentary structures (MISS). On Mars, three lithofacies of the Gillespie Lake Member sandstone display centimeter- to meter-scale structures similar in macroscopic morphology to terrestrial MISS that include ‘‘erosional remnants and pockets,’’ ‘‘mat chips,’’ ‘‘roll-ups,’’ ‘‘desiccation cracks,’’ and ‘‘gas domes.’’ The microbially induced sedimentary-like structures identified in Curiosity rover mission images do not have a random distribution. Rather, they were found to be arranged in spatial associations and temporal successions that indicate they changed over time. On Earth, if such MISS occurred with this type of spatial association and temporal succession, they would be interpreted as having recorded the growth of a microbially dominated ecosystem that thrived in pools that later dried completely: erosional pockets, mat chips, and roll-ups resulted from water eroding an ancient microbial mat–covered sedimentary surface; during the course of subsequent water recess, channels would have cut deep into the microbial mats, leaving erosional remnants behind; desiccation cracks and gas domes would have occurred during a final period of subaerial exposure of the microbial mats. In this paper, the similarities of the macroscopic morphologies, spatial associations, and temporal succession of sedimentary structures on Mars to MISS preserved on Earth has led to the following hypothesis: The sedimentary structures in the less than 3.7 billion years Gillespie Lake Member on Mars are ancient MISS produced by interactions between microbial mats and their environment. Proposed here is a strategy for detecting, identifying, confirming, and differentiating possible MISS during current and future Mars missions
Potential MISS erosional remnant on Mars (top); edge of a microbial mat–overgrown erosional remnant on Portsmouth Island, USA (middle); erosional remnant of a modern MISS on Mellum Island, Germany (bottom). Credit: Mars: NASA; Earth: Nora Noffke
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